**Reference: **
Iwasaki, Y. &
Doshi, K. Equation Model Generation: Where do Equations Come From? Knowledge Systems Laboratory, August, 1990.

**Abstract:** In all disciplines of physical and social sciences, a set of simultaneous
equations is an essential tool for describing the relations that hold among
parameters of objects and that govern their behavior over time. The two main
stages of using equations for this purpose are (1) characterization of a
system in terms of functional relations among parameters and, (2) prediction
of the system behaviour using the equations through various analytic, numeric,
or qualitative techniques. As the second stage has been studied extensively
in many fields including applied mathematics and numerical simulation, there
exist many computer programs for performing the second stage. Compared to the
second stage, much less attempts have been made to automate the first stage.
Some of the reasons for this are:
1. Model building is a process that requires a large amount of knowledge of
the domain under study.
2. Appropriate selection of parameters and equations also requires much
heuristic and commonsense knowledge in order to determine the appropriate set
of phenomena to model and the temporal grain size depending on the goal of the
analysis [4,8].
This paper discusses different types of physical knowledge required for model
generation. The paper will then focus on two issues is particular that we
have found problematic in model building.
As principles to guide the model generation process, de Kleer and Brown have
stressed the importance of locality principle and no-function-in-structure
principle [1]. Forbus has put forth the process-oriented approach [3], and
Iwasaki and Simon require that model equations to be structural [6]. After
studying the various sources of equations, we think that none of these
principles alone is sufficient nor has been articulated well enough to allow
systematic construction of models. One reason for this is that what is
considered to be processes, mechanisms, components, and connections can vary
widely from domain to domain. Also, even when one limits the problem to a
particular domain, what is an appropriate model still deppends on the levels
of abstraction and what are considered to be the primitives at each level. We
need more detailed examination of different types of physical principles
underlying equations and further refinement of these principles in order to
formulate a computation tehory of model generation.

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